1. Field of the Invention
The present invention relates to wire grid polarizers. More particularly, this invention pertains to a non-photolithographic method for fabricating wire grid polarizers.
2. Description of the Prior Art
The most significant application of polarizers at the present time is for direct view liquid crystal displays (LCD's). These are commonly employed in laptop computers, thin desktop monitors, television displays and related applications.
Various types of polarizers currently exist, including those of the dichroic, wire grid, anisotropic crystal and multi-layer thin-film type. Various considerations have led to the nearly-universal employment of polarizers of the dichroic absorption type pioneered by Edwin Land sixty years ago in LCD displays. Naturally-occurring birefringent anisotropic crystals are of insufficient size for use in the most commonly encountered LCD display sizes while the fabrication of synthetic crystals of such dimension is prohibitively expensive. Multi-layer thin-film polarizers, which rely upon the Brewster angle effect, require that a glass substrate be tilted at a nominal 45 degree angle to the display. Such a geometry can add substantial and undesirable bulkiness to the display, often ruling such technology out of potential applications.
Wire grid polarizer technology offers some inherent advantages over dichroic absorptive polarizers. While a wire grid polarizer operates by the reflection and transmission of light, and is therefore neither temperature sensitive nor does it absorb excessive amounts of energy, a dichroic absorptive polarizer operates by the selective absorption and transmission of light. As such, a dichroic based polarizer exhibits temperature sensitivity due to (a) sensitivity of the organic dye to degradation in the presence of heating and (b) thermal rearrangement (relaxation) of the polymer alignment achieved by stretching the polymer to line up the dye molecules. Such temperature sensitivity limits the types of manufacturing process that may be employed to create dichroic adsorptive polarizers. The relatively low temperature processes available are often sub-optimal in terms of yield, quality and cost.
The wire grid polarizer comprises an array of closely-spaced parallel conductive lines supported by a transmitting substrate. A perspective schematic view of such a polarizer is illustrated in FIG. 1. As can be seen, the polarizer 10 comprises an array of parallel conductive lines 12 on a transparent substrate 14. Each of the conductive lines is characterized by a thickness t, a width w and a periodic separation (or period) Λ with respect to the adjacent line(s). In operation, unpolarized light 16 is incident at an angle φ. (Note: the angle of incidence φ may be zero; that is the light 16 may be normal to the surface of the polarizer 10). A portion 18 of the incident light 16 is reflected while another portion 20 is transmitted. The reflected portion 18 is almost entirely s-polarized (electric vector parallel to the direction of the conductive lines 12) while the transmitted portion 20 is almost entirely p-polarized (electric vector perpendicular to the direction of the conductive lines 12).
Ideally, a wire grid polarizer functions as a perfect mirror for one plane of polarization (e.g. s-polarized light) and is perfectly transparent to the orthogonal plane of polarization (e.g. p-polarized light). In practice, even the most reflective metals absorb some fraction and reflect only 80 to 95 percent of incident light. Similarly, due to surface reflections, a nominally transparent substrate does not transmit 100 percent of incident light. Polarizer performance over the range of wavelengths and incidence angles of interest is characterized by the contrast ratios of the transmitted (Tp/Ts) and reflected (Rs/Rp) beams and optical efficiency (percentage of incident unpolarized light transmitted).
The overall behavior of a wire grid polarizer is determined by the relationship between (1) the center-to-center spacing, or periodicity, of the parallel conductive lines and (2) the wavelength of incident radiation. Only when the periodicity, Λ, of the lines is smaller than the wavelength of interest can the array behave like a polarizer. If the periodicity of the lines should exceed the wavelength of interest, the grid will function as a diffraction grating. Further, there exists a transition region, in which periodicity of the conductive lines falls in the range of roughly one-third to twice the wavelength of interest (i.e., 2λ≧Λ—≧λ/3). Large, abrupt changes are observed to occur in such transition region, namely increases in reflectivity coupled with corresponding decreases in reflectivity for p-polarized light. Such “Raleigh resonances” occur at one or more specific wavelengths for any given angle of incidence. As a result, wire grids having periodicities that fall within such transition region are unsuitable for use as polarizers.
Wire grid polarizers were developed for use in the millimeter-wave and microwave frequency ranges. They were initially unavailable for use in the infrared and visible wavelength ranges due to the inability of then-existing processing technologies (e.g. stretching thin wires over a mandrel) to produce parallel conducting lines of sufficiently small periodicity.
The application of photolithography overcame the problem of attaining the requisite small periodicities. See, for example, U.S. Pat. No. 4,049,944 of Garvin et al. Covering “Process for Fabricating Small Geometry Semiconductive Devices Including Integrated Components” which teaches, in part, a method for fabrication of wire grid polarizers employing holographic exposure of photolithographic materials. Other applications of photolithography in methods for forming wire grid polarizers are taught, for example, in the following United States patents: U.S. Pat. No. 6,122,103 of Perkins et al. covering “Broadband Wire Grid Polarizer For the Visible Spectrum” and U.S. Pat. No. 6,665,119 of Kurtz et al. covering “Wire Grid Polarizer”.
U.S. Pat. No. 3,046,839 of Bird et al. covering “Process For Preparing Light Polarizing Materials” and U.S. Pat. No. 4,456,515 of Krueger et al. covering “Method For Making Polarizers Comprising a Multiplicity of Parallel Electrically Conductive Strips on a Glass Carrier” disclose photolithographic processes for forming wire grid polarizers that eliminate difficult etching steps. A thin layer of metal is deposited at an oblique angle to the substrate after a photolithographic pattern of finely spaced parallel lines is fabricated directly on a transparent substrate. The oblique angle of incidence, coupled with periodic topographic steps in the resist pattern, cause the metal to accumulate primarily on the sidewalls of the pattern. When photoresist is subsequently washed away, only the thin metal lines that are attached to the substrate between ridges of photoresist and accumulated on the sidewalls of the resist pattern remain.
Photolithographic techniques for reducing the periodicity of parallel conductive lines from approximately one micrometer (limiting the resultant devices to the near IR spectrum) to approximately 0.1 micrometer (suitable for the visible spectrum) has been disclosed, for example, by Karthe (see Wolfgang Karthe, “Nanofabrication Technologies and Device Integration”, Proceedings of SPIE, vol. 2213 (July 1994), pp. 288-296).
Techniques for fabricating wire grid polarizers by methods employing photolithography face inherent and well-recognized limitations. First, the lengths of the sides of the area that can be exposed during a single exposure (and, hence, the size of the polarizer) are limited to a few inches. This is far too small for most direct view displays such as those employed in laptop computers. Secondly, the cost of photolithographic processes is rather high due to the costs of high-resolution photolithography systems, and the requisite ultra-high quality clean room facility required to house such a system.